Detecting when alternating current (AC) voltage and/or current waveforms cross through a zero current (or voltage) axis (e.g., “zero crossing”) is frequently performed, for example in electric motor control, lamp dimming, powerline communication and other applications. In motor control systems, a half-bridge circuit is frequently used to drive a switching node coupled to a load (e.g., the motor). There is difficulty in robustly detecting the zero-crossing when the switching node is noisy.
Switching node noise is a particular issue when MOSFETs are employed as power switches (e.g., in the half-bridge circuit) for a motor control. Motors tend to present an inductive load to the half-bridge circuit, and the combination of the inductive load with the capacitance of the MOSFETs can cause ringing on the switching node, making accurate detection of the zero crossing difficult.
Further, some systems monitor the current (or voltage) directly and, thus, the signal of interest is low in amplitude since the current (or voltage) is nearing zero at the detection point. Rather than directly sensing the current (or voltage), some systems monitor voltage (or current) of the switching node during “dead time” of the half-bridge circuit. Dead time is a period of time during which all the MOSFETs of the half-bridge circuit are turned off to prevent potentially damaging short circuit (or “shoot through”) conditions. However, such systems are required to sense relatively small voltages (currents), thus placing a strict requirement on the tolerances and accuracy of the components employed.
Therefore, an improved zero crossing detector is envisioned to more reliably and accurately detect zero crossings of a current (or voltage) waveform of interest.